A mark on a character's flank that somehow indicates what their special something that makes them unique. According to Cheerilee the schoolteacher pony, a cutie mark represents a special talent that the pony is both good at and enjoys doing. A character with a hammer for a cutie mark light be good at building or constructing things, or repairing objects.
A foal who has yet to find their special something may be called a blank flank, though the term is rather derogatory.... However, when they DO find that special something, and gain their cutie mark, it's traditional to have a party called a Cute-ceañera.
When to use this tag:
when a character who normally does not have a cutie mark is drawn with one
when a cutie mark is gained in an image
when cutie marks are being discussed
when a cutie mark is specifically the subject of the image
Do not use this tag on every image that has a cutie mark in it; instead, tag the specific type of cutie mark when possible (such as rainbow cutie mark) and only use this tag when a cutie mark is particularly significant to the image.